Argus Filch is the long-serving, ill-tempered caretaker of Hogwarts School of Witchcraft and Wizardry. He is a Squib, a non-magical individual born to wizarding parents, a fact that is the source of his deep-seated bitterness and resentment towards the students and the magical world he inhabits. Filch serves as a minor antagonist throughout the series, constantly patrolling the school's corridors in search of rule-breakers, assisted by his loyal cat, Mrs. Norris. He longs for the days of harsh corporal punishment and acts as a staunch, if unpleasant, enforcer of school rules, particularly under authoritarian figures like Dolores Umbridge.
Much of Filch's early life is unknown, but it was defined by his inability to perform magic. His desire to be a wizard is evident in his secret attempt to learn magic through a `Kwikspell` correspondence course, which Harry Potter discovers during his second year. Filch has been the caretaker at Hogwarts for many years, preceding the attendance of Arthur Weasley and Molly Weasley. Throughout Harry Potter's time at Hogwarts, Filch is a constant, menacing presence. In Harry Potter and the Philosopher's Stone, he catches Harry, Ron, Hermione, and Neville Longbottom out of bed at night, leading to their detention in the Forbidden Forest. During the events of Harry Potter and the Chamber of Secrets, Filch is a key figure when his cat, Mrs. Norris, becomes the first victim of the Basilisk. Devastated, he initially suspects Harry is the attacker. It is in this book that his status as a Squib is revealed to Harry. Filch's obsession with order and punishment makes him a natural ally for Dolores Umbridge in Harry Potter and the Order of the Phoenix. He is the only staff member, besides Umbridge herself, who supports her tyrannical regime, and he gleefully accepts her permission to use corporal punishment on students. He eagerly volunteers to whip the Weasley twins for creating a portable swamp, a task he is ultimately unable to perform. He wages a continuous and losing war against the castle's Poltergeist, Peeves, and the pranks of students like Fred and George Weasley, for whom he keeps a dedicated filing cabinet drawer of recorded punishments. During the Battle of Hogwarts in Harry Potter and the Deathly Hallows, Filch participates not as a combatant but by helping to evacuate younger students. True to his nature, he is later heard complaining about the mess created by the battle.
Argus Filch is described as a stooped, rheumy-eyed man with a wheezing voice and jowls that wobble when he is angry. He is almost always seen in a dusty, brown coat. His face is typically contorted with suspicion or rage. His personality is defined by bitterness, cruelty, and resentment. He harbors a deep-seated jealousy of the students' magical abilities and takes a sadistic pleasure in the thought of punishing them. His office is filled with instruments of torture like chains and manacles that he longs to use. Filch despises anything that creates mess or disorder, making Peeves and the Weasley twins his primary nemeses. The only creature for whom he shows any affection is his cat, Mrs. Norris, with whom he shares a profound bond.
As a Squib, Argus Filch possesses no personal magical abilities. His attempt to learn magic via a `Kwikspell` course was a failure, underscoring his complete lack of magical talent. Despite this, he has an unparalleled, non-magical knowledge of Hogwarts' layout, including its secret passageways and hidden corridors, a knowledge rivaled only by those who use the Marauder's Map.